In the fabrication of metallized integrated circuit substrates, various inorganic substrates such as ceramics and glass-ceramics, and organic substrates such as polyepoxides and polyimides are coated with metallic layers such as chromium and copper. For example, in the packaging of semiconductor chips, polyimide films are often coated onto substrates.
One configuration for fabricating multilayer substrates for mounting chips employs an insulating substrate of a ceramic material onto which is deposited a pattern of metallic conductors. Usually, the conductors are three layers of metal, being a layer of chromium, followed by a layer of copper, followed by an overlying layer of chromium. On top of the metallized ceramic substrate is placed a layer or film of a polyimide and on top of the polyimide a second layer of a pattern of conductors such as chromium-copper-chromium is provided.
Adhesion between the chromium and dielectric layers has not been entirely satisfactory and has been the subject of research with a view towards enhanced adhesion. For instance, it has been suggested in U.S. Pat. No. 4,386,116 to Nair, et al. and assigned to International Business Machines Corporation, the assignee of the present application, disclosure of which is incorporated herein by reference, to have copper as the metal contacting polyimide on the second or subsequent layers. The copper also contacts the copper top layer of the first layer at the base of the vias in the polyimide, the top chromium having been removed at those locations.
For instance, it is necessary to electrically contact some of the conductors in the upper or second layer of metallization to some of the conductors on the lower or first layer of metallization. In order to do so, the polyimide must be selectively etched to form the desired vias therein to allow for metal connection between the upper and lower levels of metallization.
However, the adhesion between the copper and polyimide is not entirely satisfactory and could stand a degree of improvement. In particular, problems have occurred at the copper-polyimide interface resulting in lifting up of the copper line from the underlying polyimide substrate rendering the carrier unsuitable for its intended purpose.
Another use would be as a dielectric and/or circuit carrier for flexible circuits. This would involve, for example, spray coating or roller coating polyamic acid onto a sheet of metal such as stainless steel, copper, copper-metal composite such as copper-clad Invar (a trademark of International Nickel Company), or aluminum. The film is then thermally cured or imidized, resulting in a film which is fully or substantially fully thermally cured. The metal which the polyimide is on can be imaged, removed, or maintained. On top of the polyimide, three layers of metal are deposited such as by either evaporation or sputtering. The conductors are chromium or nickel, followed by a layer of copper, followed by a layer of chromium or nickel. By means of photolithographic operations, this metal is imaged into circuits. Depending on the use of the circuit, the polyimide may or may not be imaged either before or after the formation of the circuit.
Accordingly, the importance of improving the adhesion between such substrates and the metallic layers cannot be overemphasized.